In recent years, development of wireless communication systems including a mobile telephone, a wireless LAN (Local Area Network), and ITS (Intelligent Transport Systems) has caused the need for protection of communication information and for prevention of radio interference and false communication. When the main purpose is protection of communication information, each of indoor and outdoor radio waves is isolated by using an electromagnetic wave-shielding material so as to shield each of a foreign radio wave and a radiated wave from a communication device. However, in this case, a radiated wave from a communication device itself remains in a room due to reflection, and this reflected wave and a desired communication radio wave can be interfered, thereby causing deterioration of communication quality. In order to prevent this communication failure including deterioration of communication quality, and radio interference and false communication, a wave absorber is used, in which radio waves are absorbed and converted into heat.
Examples of a wave absorber generally include a material in which electromagnetic wave energy can be converted into heat and consumed. Examples of this material include a material having magnetic loss, dielectric loss, or ohmic loss. Specifically, proposed examples of a wave absorber include a wave absorber produced by mixing and dispersing a magnetic powder such as ferrite or a soft magnetic metal in an insulative matrix such as a rubber or a plastic; and molding it to be sheet-shaped or block-shaped (for example, see Patent Document 1.).
Also, specifically proposed examples of a wave absorber include a wave absorber produced by impregnating a dielectric loss powder such as carbon black in foamed polyurethane; and molding it to be pyramid-shaped or wedge-shaped (for example, see Patent Document 2.).
Also, specifically proposed examples of a wave absorber include a wave absorber produced by placing resistance films with about 377 Ω in a position being λ/4 (λ: a wavelength of a radio wave at a certain frequency) apart from the reflector (for example, see Patent Document 3.). This wave absorber is called a λ/4 type, and 377 Ω is the characteristic impedance in free space.
Also, specifically proposed examples of a wave absorber include a wave absorber produced by forming periodic conductive patterns on the surface of an absorber (for example, see Patent Document 4.). This wave absorber is proposed to be thin-sized and reduced in weight.
Furthermore, specifically proposed examples of a wave absorber include a wave absorber produced by forming periodic conducting loops on the surface of an absorber (for example, see Patent Document 5.). This wave absorber is proposed to be thin-sized, reduced in weight, and improved in a wave absorption property from an oblique direction.
However, in the wave absorber described in Patent Document 1, which is produced by mixing and dispersing a magnetic powder such as ferrite or a soft magnetic metal in an insulative matrix such as a rubber or a plastic followed by molding, it is possible to form a relatively thin absorber, but there are the drawbacks that a more thickness is necessary when high-performance wave absorption is required, and there is a problem in that the weight increases due to use of material of high specific gravity.
Also, in a wave absorber described in Patent Document 2, which is produced by impregnating a dielectric loss powder such as carbon black in foamed polyurethane followed by molding, the absorption performance is basically dependent on thickness. Therefore, there are problems in that the wave absorber should be pyramid-shaped or wedge-shaped or that the thickness should be large along the absorption direction, in order to obtain the desired performance.
Therefore, in order to obtain desired performance, the wave absorber should be pyramid-shaped or wedge-shaped, or the thickness should be large along the absorption direction. These are problems.
Also, in a wave absorber described in Patent Document 3, which is produced by placing resistance films with about 377 Ω in the position being λ/4 apart from the reflector, a transparent wave absorber can be produced by using optically transparent resistance films. However, a wave absorber described in Patent Document 3 requires a thickness of λ/4 at a certain frequency in principle, and has a problem in that a radio wave absorption property varies according to the incident angle of the wave.
In addition, Patent Document 4 discloses a wave absorber including a periodic loop pattern, in which plural conductive patterns are placed regularly, and an intermediate resin layer and a conductive reflection layer. The resin layer contains loss materials. Also, Patent Document 4 discloses that this wave absorber is thin-sized and reduced in weight in comparison with conventional ones. However, this wave absorber has a problem in that a radio wave absorption property (frequency) varies according to the incident angle of the wave as in the case of the λ/4 type.
Furthermore, Patent Document 5 discloses a wave absorber including periodic conducting loops are placed regularly, an intermediate resin layer and a conductive reflection layer, whose thickness is 0.027 times or more of a wavelength to be absorbed. Also, Patent Document 5 discloses that this wave absorber is thin-sized and reduced in weight in comparison with conventional ones. However, in a wave absorber having a structure in which patterns with a single size are periodically arranged, a frequency bandwidth is so restricted as to become quite narrow although variation of wave absorption properties (frequency) according to incident angle is prevented. Therefore, the wave absorber has a problem in property variation during production.
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2001-308584
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. Hei 10-051180
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. Hei 05-335832
[Patent Document 4] Japanese Patent No. 3209453
[Patent Document 5] Japanese Unexamined Patent Application, First Publication No. 2001-352191